1. Field of the Invention
The invention relates to a spindle device used for, for example, a machine tool.
2. Description of the Related Art
A spindle of a spindle device for a machine tool is supported by rolling bearings having high stiffness and high precision in order to increase productivity. However, for example, in a machining center, workpieces are machined with the use of various tools such as a milling cutter, an endmill and a boring tool. Therefore, generated cutting resistance varies depending on the tools. Further, many factors such as stiffness and weights of tools, rotation speeds of spindles, feedrates and feed amounts (hereinafter, referred also to as “depths of cut” where appropriate) vary. Accordingly, in a spindle that is supported by rolling bearings having high stiffness, so-called chattering may be generated if some types of tools are used.
To address this problem, for example, Japanese Patent Application Publication No. 2007-44852 (JP-A-2007-44852) suggests a spindle device with which the generation of chattering during machining is suppressed by changing the machining conditions. That is, the relationship between the rotation speed of a spindle and the upper limit of feed amounts of a tool (depths of cut formed by a tool), at which chattering is not generated, is obtained in advance, and the rotation speed of the spindle or the feed amount of the tool is changed to suppress the generation of chattering during machining. However, the relationship between the rotation speed of the spindle and the upper limit of feed amounts varies depending on, for example, types of tools and materials of workpieces. In addition, this relationship varies depending on the degree of deterioration, for example, abrasion of the tool. Therefore, the machining conditions are changed based on skills and experiences of workers in many cases. Accordingly, it is not easy to appropriately change the machining conditions.
Regenerative chatter, which is one type of chattering, is generated due to natural vibration of a mechanical system, and the generation limit of the regenerative chatter is determined by the dynamic stiffness (compliance). Therefore, it is difficult to greatly change the generation limit of the regenerative chatter by just changing the stiffness, and a damping property needs to be provided. Therefore, Japanese Patent Application Publication No. 2004-150563 suggests a spindle device in which a rear end portion of a spindle is supported by a bearing that has a damper portion to provide a damping property. With this configuration, the generation of vibration due to resonance is suppressed. Japanese Patent Application Publication No. 6-8005 (JP-A-6-8005) suggests a spindle device in which a front end portion of a spindle is supported by a hydrostatic bearing formed of a static pad and an orifice to provide a damping property. The vibration of the spindle is detected by a vibration sensor, and the hydraulic pressure that is supplied to the hydrostatic bearing is regulated by a pressure-regulating valve. With this configuration, the generation of chattering during machining is suppressed.
In the spindle device described in JP-A-2004-150563, the damper portion damps the vibration due to resonance at a certain frequency, that is, the vibration due to resonance generated by a certain type of tool. Therefore, it is difficult to suppress the generation of vibrations due to resonances of various tools. In the spindle device described in JP-A-6-8005, the hydraulic pressure that is supplied to the hydrostatic bearing is regulated based only on the vibration of the spindle. Therefore, stabilities of the machining condition and the spindle state are not secured. Accordingly, there is a limit to improvement of the machining accuracy. Further, the damping property can be changed only roughly because only the hydraulic pressure that is supplied to the hydrostatic bearing is regulated. Therefore, it is difficult to suppress the generation of chattering of various tools.